This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-022651, filed Jan. 31, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a cathode ray tube apparatus comprising an electron gun assembly which emits one or more electron beams, and particularly to a cathode ray tube apparatus in which focus characteristics of the electron beam or beams are improved so that high resolution is obtained for the entire screen.
In general, in a color cathode ray tube apparatus, the three-electron beams emitted from an electron gun assembly is deflected by horizontal and vertical deflection magnetic fields. The deflected beams are oriented to a fluorescent screen made of a three-color fluorescent layers which are scanned horizontally and vertically by the electron beams so that a color image is displayed on the fluorescent screen.
Particularly, in this cathode ray tube apparatus, there is a trend as follows. That is, the electron gun assembly is constructed as an in-line type electron gun assembly which emits three electron beams arranged in line and including one center beam and a pair of side beams which penetrate in one same horizontal plane. On the other hand, its deflection yoke generates a horizontal deflection magnetic field of a pincushion type and a vertical deflection magnetic field of a barrel type, thereby to converge the three electron beams emitted from the electron gun assembly and arranged in line, onto a phosphor screen.
In this kind of cathode ray tube apparatus, the deflection magnetic field described above is not uniform, and therefore, the electron beam spot receives a diverging effect in the horizontal direction, causing an under-focused state even if the electron beam spot formed on the center part of the phosphor screen is a true circle. In the vertical direction, the electron-beam spot receives a focusing effect, causing an over-focused state.
Further, the distance from the electron gun assembly to the phosphor screen increases with the deflection amount of the electron beam. Accordingly, even if the electron beams spot is formed to be a true circle at the center part of the phosphor screen, the beam spot becomes over-focused at the peripheral portion of the phosphor screen.
As a result of this, the electron spot at the periphery part of the phosphor screen becomes remarkably over-focused in the vertical direction due to the two effects described above, and the above two effects compensate for each other in the horizontal direction to cause a substantially focused state. That is, in the peripheral part of the phosphor screen, astigmatic aberration caused due to a difference in the focus state between the vertical and horizontal directions. As shown in FIG. 1, the electron beam spot 2 is deformed into an asymmetric shape composed of a core part 3 as a high-luminance part and a halo part 4 as a low-luminance part, so that the resolution is remarkably degraded at the peripheral part of the phosphor screen. In addition, deflection aberration received by the electron beam increases as the scale of the cathode ray tube apparatus increases and the deflection angle increases. In this case, the resolution at the peripheral part of the phosphor screen is deteriorated much more.
To modify the electron beam spot, it is also important that the electrode forming the main lens of the electron gun assembly is formed with a large hole diameter so as to reduce the spherical aberration. Therefore, the mutual distance between the three electron beams must be set large. However, if the electron gun assembly is designed to have a large mutual distance between the three electron beams, there is a problem that the convergence characteristic of the three electron beams is deteriorated. Also, the hole diameter of the electrode forming the main lens part is limited by the inner diameter of the neck where electron gun assembly is provided. That is in order to attain an excellent resolution of the color cathode ray tube apparatus, it is necessary to enlarge the effective diameter of the main lens without increasing the mutual distance between the three electron beams, and to improve deformation of the electron beam spot at the peripheral part of the screen.
As a method of achieving improvements for an enlarged diameter and a deflection deformation of the main lens, Japanese Patent Application KOKAI Publication No. 64-38947 proposes an electron gun assembly having a structure as follows. In this electron gun assembly, the main lens is comprised of a focus electrode G5, two intermediate electrodes Gm1 and Gm2, and a final acceleration electrode G6. In the electron gun assembly shown in these FIGS. 2A and 2B, a high voltage applied to the final acceleration electrode G6 is divided by a resistor T provided along the electrode of the electron gun assembly, and predetermined divided voltages are applied to the intermediate electrodes Gm1 and Gm 2. In addition, a dynamic voltage having a parabola shape which changes in accordance with deflection of the electron beam is applied to the focus electrode G5, superposed into a constant direct current voltage. All the beam-passing holes of the focus electrode G5, intermediate electrodes Gm1 and Gm 2, and the final acceleration electrode G6 are each formed to be a true circular shape. In addition, no side wall part, i.e., no bar ring is formed on the side surface of each electron-beam-passing hole, in the focus electrode G5 and the final acceleration electrode G6. Therefore, an electric field common to three electron beams is formed in the horizontal direction inside the focus electrode G5 and the final acceleration electrode G6. As a result of this, a first quadrapole lens having a strong focusing effect in the relatively vertical direction is formed near the focus electrode G5, and a second quadrapole lens having a strong diverging effect in the relatively vertical direction is formed near the final acceleration electrode G6.
Accordingly, in the electron gun assembly having a structure as described above, an enhanced electric field lens in which the main lens is enhanced by the intermediate electrodes Gm1 and Gm2 can be formed. Further, if the electron beam is deflected at the peripheral parts of the screen, the focus electrode G5 is supplied with a higher voltage (dynamic voltage) in accordance with the deflection of the electron beam, so that the voltage difference is decreased between the focus electrode G5 and the intermediate electrode Gm1. Therefore, the effect of the first quadrapole lens is weakened. Accordingly, the electron beam is diverged in the vertical direction while the focused state of the electron beam is not substantially changed in the horizontal direction. As a result, it is possible to compensate for the over-focusing in the vertical direction, which is caused by the non-uniform magnetic field generated from the deflection yoke. In the horizontal direction, deterioration of the magnification is smaller compared with a dynamic electron gun assembly in which a quadrapole lens is provided in the side closer to the cathode side than the main lens. Therefore, the electron-beam spot can have a smaller diameter.
By the electron gun assembly having a structure as described above, it is possible to solve two problems, i.e., the enlarged effective diameter described above and improvement concerning the deterioration of the resolution due to deflection aberration described above.
However, in case of the electron gun assembly having the structure described above, no side wall part (bar ring) is formed on the side surface of each of the electron-beam-passing holes, and therefore, the effective diameter is smaller in the vertical direction than in the horizontal direction. Consequently, the lens magnification and spherical aberration are enlarged so much that the diameter of the electron beam spot in the vertical direction becomes larger than that of the electron beam spot in the horizontal direction. As a result, the resolution is deteriorated at the center part of the screen. In particular, if the size and deflection angle of the cathode ray tube apparatus are large, it is necessary to strengthen the effect of the first quadrapole lens. In this case, the true circular shape of each hole formed in the focus electrode G5 and the final acceleration electrode G6 may be changed into a laterally elongated shape. However, the effective diameter is much more reduced in the vertical direction, so that the spherical aberration in the vertical direction is more increased, and the electron beam spot is much more elongated in the longitudinal direction at the center part of the screen. Consequently, the resolution is remarkably deteriorated in the center part of the screen.
As described above, in order to obtain an excellent resolution of the cathode ray tube apparatus, it is necessary to enlarge the effective diameter of the main lens without increasing a large mutual distance between three electron beams, and to improve deformation of the electron beam spots at the peripheral part of the screen.
As an electron gun assembly which achieves an enlargement of the effective diameter of the main lens and the improvement of the deflection deformation, there has been an electron gun assembly as follows. In this gun, the main lens is constructed by a focus electrode, an intermediate electrode applied with a desired voltage divided by a resistor incorporated in the tube, and a final acceleration electrode. Near the focus electrode, asymmetric focusing electric field which provides a strong focusing effect relatively in the vertical direction is created near the focus electrode, and an asymmetric diverging electric field which provides a strong diverging effect relatively in the vertical direction is created near the final acceleration electrode. The asymmetric focusing electric field and the asymmetric diverging electric field are separated substantially by the intermediate electrode, so that a dynamic voltage which changes in synchronization with deflection of the electron beam is supplied to the focus electrode.
However, by merely adopting this structure, the lens magnification and the spherical aberration are increased much more in the vertical direction than in the horizontal direction, and the electron beam spot diameter becomes larger in the vertical direction than in the horizontal direction, so that the resolution is deteriorated at the center part of the screen. In particular, if the size and deflection angle of the cathode ray tube apparatus are large, the lens magnification and the spherical aberration in the vertical direction are increased much more, resulting in a problem that the resolution is remarkably deteriorated.
An object of the present invention is to provide a cathode ray tube apparatus comprising an electron gun assembly in which the diameter of the electron beam spot is small and uniform throughout the entire area of the phosphor screen, so that the resolution of the cathode ray tube apparatus can be improved.
According to the present invention, there is provided a cathode ray tube apparatus comprising: an envelope having a screen; and an electron gun assembly constructed by a cathode for emitting an electron beam, and a main lens including a focus electrode, an intermediate electrode, and a final acceleration electrode, to focus the electron beam emitted toward the screen, the intermediate electrode being provided between the focus electrode and the final acceleration electrode, and wherein the main lens includes a focusing area positioned in a side of the focus electrode and having a focusing force, and a diverging area positioned in a side of the final acceleration electrode, having a diverging force, and being continuous to the focusing area, that the intermediate electrode has a hole having a non-circular shape for allowing the electron beam to pass and is provided in the focusing area in the side of the focus electrode, that a focusing force curve expressing a focusing force along a tube-axis direction of the cathode ray tube apparatus in the focusing area has at least two convex parts respectively being at first and second levels, and a concave part provided between the convex parts and being at a third level sufficiently lower than focusing forces of the first and second levels, that the third level is set to a lowermost level at which a focusing or diverging force is not substantially effected on the electron beam or a focusing or diverging force is sufficiently small even if it is effected, that the intermediate electrode is positioned near an area of the lowermost level, and that at least one electrode constructing the main lens is applied with a dynamic voltage which changes in synchronization with deflection of the electron beam.
Also, according to the present invention, there is provided a cathode ray tube apparatus having the structure as described above and wherein the focusing force or diverging force at the lowermost level has an absolute value which is substantially equal to or less than half of a uppermost focusing force which can be effected by the main lens.
Further, according to the present invention, there is provided a cathode ray tube apparatus having the structure as described above and wherein an asymmetric intermediate electrode is provided at or near a boundary part between the large focusing area positioned in the side of the focus electrode and the large diverging area positioned in the side of the final acceleration electrode.
Further, according to the present invention, there is provided a cathode ray tube apparatus having the structure as described above and wherein a quadrapole lens is provided in a side of the cathode of the main lens, and a dynamic voltage which changes in synchronization with deflection of the electron beam is applied to an electrode constructing the quadrapole electrode.
According to the present invention, there is provided a cathode ray tube apparatus comprising: an envelope having a screen; and an electron gun assembly constructed by a cathode for emitting an electron beam, and a main lens including a focus electrode, an intermediate electrode, and a final acceleration electrode, to focus the electron beam emitted toward the screen, the intermediate electrode being provided between the focus electrode and the final acceleration electrode, and wherein the main lens includes a focusing area positioned in a side of the focus electrode and having a focusing force, and a diverging area positioned in a side of the final acceleration electrode, having a diverging force, and being continuous to the focusing area, that an intermediate electrode having a non-circular shaped hole for allowing the electron beam to pass is provided in the diverging area in the side of the final acceleration electrode, that a focusing force curve expressing a focusing force along a tube-axis direction of the cathode ray tube apparatus in the focusing area has a convex part, that the curve at the convex part includes a part being at a uppermost level,.that the uppermost level is set such that a focusing or diverging force is not substantially effected on the electron beam or a focusing or diverging force is sufficiently small even if it is effected, that the intermediate electrode is positioned near the part of the uppermost level, and that the electrode constructing the main lens is applied with a dynamic voltage which changes in synchronization with deflection of the electron beam.
Further, according to the present invention, there is provided a cathode ray tube apparatus having the structure as described above and wherein the focusing force or diverging force at the uppermost level has an absolute value which is substantially equal to or less than half of a uppermost diverging force which can be effected by the main lens.
Further, according to the present invention, there is provided a cathode ray tube apparatus comprising: an envelope having a screen; and an electron gun assembly constructed by a cathode for emitting an electron beam, and a main lens including a focus electrode, an intermediate electrode, and a final acceleration electrode, to focus the electron beam emitted toward the screen, the intermediate electrode being provided between the focus electrode and the final acceleration electrode, and wherein the main lens includes a focusing area positioned in a side of the focus electrode and having a focusing force, and a diverging area positioned in a side of the final acceleration electrode, having a diverging force, and being continuous to the focusing area, that an intermediate electrode having a non-circular shaped hole for allowing the electron beam to pass is provided in the focusing area in the side of the focus electrode and in the diverging area in the side of the final acceleration electrode, that a focusing force curve expressing a focusing force along a tube-axis direction of the cathode ray tube apparatus in the focusing area has at least two convex parts respectively being at first and second levels and a concave part provided between the convex parts and being at a third level which is sufficiently smaller than focusing forces of the first and second levels, that the third level is set to a lowermost level at which a focusing or diverging force is not substantially effected on the electron beam or a focusing or diverging force is sufficiently small even if it is effected, that the intermediate electrode having the non-circular shaped hole is provided near the part of the lowermost level, that the intermediate electrode is provided in the diverging area in the side of the final acceleration electrode, that the focusing force curve expressing the focusing force along the tube axis direction of the cathode ray tube apparatus in the diverging area is formed to be a convex part, that the curve at the convex part has a part being at a uppermost level, that the uppermost level is set such that a focusing or diverging force is not substantially effected on the electron beam or a focusing or diverging force is sufficiently small even if it is effected, the intermediate electrode having the non-circular shaped hole is provided near the part of the uppermost level, and that the electrode constructing the main lens is applied with a dynamic voltage which changes in synchronization with deflection of the electron beam.
Also, according to the present invention, there is provided a cathode ray tube apparatus having the structure as described above and wherein an asymmetric intermediate electrode is provided at or near a boundary of the large focusing area positioned in the side of the focus electrode and the large diverging area positioned in the final acceleration electrode.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.